1. Field of the Invention
The present invention generally relates to free-space optical (FSO) communications systems, and, more specifically, to an apparatus and method for efficiently combining a plurality of single-mode or multi-mode fiber-coupled optical beams into single fiber-coupled multimode optical beam.
2. Background Information
With the increasing popularity of wide area networks (WANs), such as the Internet and/or the World Wide Web, network growth and traffic has exploded in recent years. Network users continue to demand faster networks and more access for both businesses and consumers. As network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to present day hardwired or fiber network solutions is the use of wireless optical communications. Wireless optical communications utilize point-to-point communications through free-space and therefore do not require the routing of cables or fibers between locations. Wireless optical communications are also known as free-space optical (FSO) or atmospheric optical communications. In a typical free-space optical communication system, a modulated beam of light is directed through free-space from a transmitter at a first location to a receiver at a second location. Data or information is encoded into the beam of light by means of the modulation. Once collected by the receiver, the modulated beam of light is demodulated and corresponding data and information may then be extracted. This scheme enables data and information to be transmitted through free-space from the first location to the second location.
In some instances, it is desired to provide redundant FSO systems to maintain uptime in the event of a component failure. One conventional approach is to provide two or more duplicate FSO communication channels, wherein the sending and receiving terminals include separate laser sources, modulator electronics, transmission optics, receive optics, and demodulators for each channel. In such configurations, instances of the same data stream are concurrently transferred between the sending and receiving terminals. This is a very costly approach, however.
Another desire is to increase the bandwidth between terminals in an FSO installation. One approach is to use multiple separate channels using a configuration similar to that described above for the conventional redundant FSO system. As before, this approach is costly.
Another approach is to use wavelength division multiplexed (WDM) optical signals. In this instance, a plurality of multiplexed optical signals are superimposed on one another and transmitted via a single modulated beam comprising a plurality of wavelengths, wherein each multiplexed optical signal has a separate wavelength. A primary requirement in WDM systems is a means for combining multiple optical signals into a single beam. One conventional approach is to use a multiple-IN/single-OUT star-coupler just before a transmit aperture, with one or more high-power amplifiers at an input side of the coupler being used to amplify the signals that are input into the coupler. However, use of such a coupler results in significant power loss on the output side of the coupler. For example, a 4-to-1 coupler results in a 6-dB (or ¾) drop in power, after amplification. Thus, a large portion of the amplification is lost via the coupler.
Another means for combining optical signals comprises a high density chip-to-chip optical interconnection based on a microelectromechanical (MEMS) device, as disclosed in an article entitled, “Design and Analysis of Micromechanical Tunable Interferometers for WDM Free-Space Optical Interconnection,” published in Journal of Lightwave Technology, Volume 17, Issue 1. Channel connection is made by wavelength matching between tunable light sources and detectors using micromechanical Fabry-Perot interferometers. Under this scheme, an arbitrary number of incoming optical signals can be combined into a single outgoing signal. A significant drawback of this scheme is that it only supports optical signals with very low power levels.